1. Field of the Invention
The present invention relates to a display device, particularly, to a display device using a thin film transistor formed on the transparent substrate such as glass or plastic. Further, it relates to an electronic apparatus using the display device.
2. Description of the Related Art
In recent years, a cellular phone has been becoming popular by developing communication technology. In future, electrical transmission of moving pictures and transmission of a large quantity of information will be expected. With being lightened, a mobile personal computer is into production. An information device called a personal digital assistant (PDA) developed from electrical books is produced and becoming popular. With developing a display device and the like, most of such portable information devices are equipped with flat displays.
The latest technology aims at using an active matrix display device as a display device used in the portable information device.
In the active matrix display device, TFTs (thin film transistors) are provided in correspondence with respective pixels to control pictures. The active matrix display device has an advantages that the high definition of images is possible, the improvement of image quality is possible, the correspondence to moving image is possible, and the like, compared to a passive matrix display device. Therefore, the display device of the portable information device will be changed from a passive matrix type to an active matrix type.
Above all, a display device using low-temperature polysilicon has been production in recent years. In the low-temperature polysilicon technology, the driver circuit using TFTs can be formed simultaneously in the periphery of a pixel portion in addition to a pixel TFT that constitutes a pixel. Thereby, the low-temperature polysilicon technology can contribute to miniaturization of devices and low power consumption. Accordingly, the low-temperature polysilicon device becomes indispensable to the display device of the mobile device that has been widely applied to various fields in recent years.
In recent years, the development of a display device using an organic electro luminescence element (OLED) has been becoming more and more active. Hereinafter, the OLED includes both the OLED using luminescence from singlet exciton (fluorescence) and the OLED using luminescence from triplet exciton (phosphorescence) here. In this specification, the OLED is described as an example of a light emitting element, however, another light emitting elements can be used.
The OLED has a structure in which an OLED layer is interposed between a pair of electrodes (an anode and a cathode), and usually has a laminated structure. Representatively, there is a laminated structure which is called “hole transporting layer, light emitting layer, electron transporting layer”, proposed by Tang et al. of Kodak Eastman Company.
Other structures may also be adopted, such as a structure in which “a hole injecting layer, a hole transporting layer, a light emitting layer and an electron transporting layer” are stacked on an anode in order, or a structure in which “a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer” are laminated on an anode in order. The light emitting layer may also be doped with a fluorescent pigment or the like.
In this specification, all layers provided between a cathode and an anode are herein generically called “OLED layer”. Accordingly, all the aforementioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer and electron injecting layer are encompassed in the OLED layer. A light emitting element constituted of an anode, an OLED layer, and a cathode is called “OLED”.
FIG. 3 shows an example of the construction of a pixel portion of an active matrix type OLED display device. A gate signal line (G1 to Gy) to which a selection signal is to be inputted from a gate signal line driver circuit is connected to a gate electrode of a switching TFT 301 which is provided in each pixel of the pixel portion. Either one of source and drain regions of the switching TFT 301 provided in each pixel is connected to a source signal line (S1 to Sx) to which a signal is to be inputted from a source signal line driver circuit, while the other is connected to a gate electrode of an OLED driving TFT 302 and to either one of electrodes of a capacitor 303 which is provided in each pixel. The other electrode of the capacitor 303 is connected to a power supply line (V1 to Vx). Either one of source and drain regions of the OLED driving TFT 302 provided in each pixel is connected to the power supply line (V1 to Vx), while the other is connected to one of electrodes of the OLED 304 provided in each pixel.
The OLED 304 has an anode, a cathode and an OLED layer provided between the anode and the cathode. If the anode of the OLED 304 is connected to the source region or the drain region of the OLED driving TFT 302, the anode and the cathode of the OLED 304 become a pixel electrode and a counter electrode, respectively. Contrarily, if the cathode of the OLED 304 is connected to the source region or the drain region of the OLED driving TFT 302, the cathode and the anode of the OLED 304 become a pixel electrode and a counter electrode, respectively.
Incidentally, the potential of the counter electrode is herein called “counter potential”, and a power source for applying the counter potential to the counter electrode is herein called “counter power source”. The difference between the potential of the pixel electrode and the potential of the counter electrode is an OLED driving voltage, and the OLED driving voltage is applied to the OLED layer.
As a gray scale display method for the above-described OLED display device, there are an analog gray scale method and a time gray scale method.
First, the analog gray scale method for the OLED display device will be described below. FIG. 4 is a timing chart showing the case driving the display device shown in FIG. 3 by the analog gray scale method. The period that starts when one gate signal is selected and finishes when the next gate signal line is selected is herein called “one line period (L)”. The period that starts when one image is selected and finishes when the next image is selected corresponds to one frame period. In the case of the OLED display device shown in FIG. 5, the number of gate signal lines is “y”, and y-number of line periods (L1 to Ly) are provided in one frame period.
As resolution of the OLED display device becomes higher, the number of line periods for one frame period becomes larger, and the driver circuit of the OLED display device must be driven at a higher frequency.
The power source lines (V1 to Vx) are kept at a constant voltage (power source potential). In addition, the counter potential is kept constant. The counter potential has a potential difference from the power source potential so that the OLED emit light.
In the first line period (L1), a selection signal from the gate signal line driver circuit is inputted to the gate signal line G1. Then, analog video signals are inputted to the source signal lines (S1 to Sx) in order.
Since all the switching TFTs 301 connected to the gate signal line GI are turned on, the analog video signals inputted to the source signal lines (S1 to Sx) are respectively inputted to the gate electrodes of the OLED driving TFTs 302 via the switching TFTs 301.
According to the potential of the analog video signal inputted into the pixel when the switching TFT 301 is turned on, the gate voltage of the OLED driving TFT 302 varies. At this time, the drain current of the OLED driving TFT 302 to the gate voltage is determined at a 1-to-1 ratio in accordance with the Id-Vg characteristic of the OLED driving TFT 302. Specifically, according to the potential of the analog video signal inputted to the gate electrode of the OLED driving TFT 302, the potential of the drain region of the OLED driving TFT 302 (an OLED driving voltage which is corresponding to the on state) is determined, a predetermined drain current flows into the OLED 304, and the OLED 304 emits light at the amount of emission which is corresponding to the amount of the drain current.
When the above-described operation is repeated until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx), the first line period (L1) terminates. Incidentally, one line period may also be defined as the sum of the period required until the termination of inputting the analog video signals to the respective source signal lines (S1 to Sx) and a horizontal retrace period. Then, the second line period (L2) starts, and a selection signal is inputted to the gate signal line G2. Similarly to the first line period (L1), analog video signals are inputted to the source signal lines (S1 to Sx) in order.
When selection signals are inputted to all the gate signal lines (Gl to Gy), all the line periods (L1 to Ly) terminate. When all the line periods (L1 to Ly) terminate, one frame period terminates. During one frame period, all the pixels perform displaying and one image is formed. Incidentally, one frame period may also be defined as the sum of all the line periods (L1 to Ly) and a vertical retrace period.
As described above, the amount of emission of the OLED is controlled by the analog video signal, and gray scale display is provided by controlling the amount of emission. In the analog gray scale method, gray scale display is carried out by the variation in the potentials of the respective analog video signals inputted to the source signal lines.
The time gray scale method will be described below.
In the time gray scale method, digital signals are inputted to pixels to select a emitting state or a non-emitting state of the respective OLED, whereby gray scales are represented by accumulating periods per frame period during which each of the OLED emits.
In the following description, 2n gray scales (n is a natural number) are represented. FIG. 5 is a timing chart showing the case of driving the display device shown in FIG. 3 by the time gray scale method. One frame period is divided into n-number of sub-frame periods (SF1 to SFn). Incidentally, the period for which all the pixels in the pixel portion display one image is called “one frame period (F)”. Plural periods into which one frame period is divided are called “sub-frame periods”, respectively. As the number of gray scales increases, the number into which one frame period is divided also increases, and the driver circuit of the OLED display device must be driven at a higher frequency.
One sub-frame period is divided into a write period (Ta) and a display period (Ts). The write period is a period for which digital signals are inputted to all the pixels during one sub-frame period, and the display period (also called “lighting period”) is a period for which the respective OLED are in an emitting state or a non-emitting state in accordance with the input digital signals, thereby perform displaying.
The OLED driving voltage shown in FIG. 5 represents the OLED driving voltage of an OLED for which the emitting state is selected. Specifically, the OLED driving voltage (FIG. 3) of the OLED for which the emitting state is selected is 0 V during the write period, and has a magnitude which enables the OLED to emit light, during the display period.
The counter potential is controlled by an external switch (not shown) so that the counter potential is kept at approximately the same level as the power source potential during the write period, and has, during the display period, a potential difference from the power source potential to so that the OLED emits light.
The write period and the display period of each sub-frame period will first be described in detail with reference to FIGS. 3 and 5, and subsequently, the time gray scale method will be described.
First, a gate signal is inputted to the gate signal line G1, and all the switching TFTs 301 connected to the gate signal line G1 are turned on. Then, digital signals are inputted to the source signal lines (S1 to Sx) in order. The counter potential is kept at the same level as the potential of the power supply lines (V1 to Vx) (power source potential). Each of the digital signals has information of “0 ” or “1”, that is, each of the digital signals of “0” or “1” has a voltage of high level or low level.
Then, the digital signals inputted to the source signal lines (SI to Sx) are respectively inputted to the gate electrodes of the OLED driving TFTs 302 via the switching TFTs 301 which are in the on state. The respective digital signals are also inputted to the capacitors 303 and stored.
Then, the above-described operation is repeated by inputting gate signals to the respective gate signal lines (G2 to Gy) in order, whereby digital signals are inputted to all the pixels and the input digital signal is held in each of the pixels. The period required until the digital signals are inputted to all the pixels is called “write period”.
When the digital signals are inputted to all the pixels, all the switching TFTs 301 are turned off. Thus, the external switch (not shown) connected to the counter electrode causes the counter potential to vary so that a potential difference that enables the OLED 304 to emit light is produced between the counter potential and the power source potential.
In the case where the digital signals have information of “0”, the OLED driving TFTs 302 are turned off and the OLED 304 do not emit light. Contrarily, in the case where the digital signals have information of “1”, the OLED driving TFTs 302 are turned on. Consequently, the pixel electrodes of the respective OLED 304 are kept at approximately the same potential as the power source potential, and the OLED 304 emit light. In this manner, the emitting state or the non-emitting state of the OLED 304 is selected in accordance with the information of the digital signals, and all the pixels perform displaying at the same time. When all the pixels perform display, an image is formed. The period for which the pixels perform displaying is called “display period”.
The lengths of the write periods (Ta1 to Tan) of all the n-number of sub-frame periods (SF1 to SFn) are the same. The display periods (Ts) of the respective sub-frame periods (SF1 to SFn) are denoted by Ts1 to Tsn.
The lengths of the respective display periods are set to become Ts1:Ts2:Ts3: . . . : Ts(n−1):Tsn=20:2−1:22: . . . :2−(n−2):2−(n−1), respectively. By combining desired ones of these display periods, it is possible to provide a desired gray scale of 2n gray scales.
The display period is any one of Ts1 to Tsn. Here, it is assumed that predetermined pixels are turned on for Ts1.
Then, when the next write period starts and data signals are inputted to all the pixels, the next display period starts. At this time, the display period is any one of Ts2 to Tsn. Here, it is assumed that predetermined pixels are turned on for Ts2.
The same operation is repeated as to the remaining (n−2)-number of sub-frames, whereby the display periods are set as Ts3, Ts4, . . . , Tsn in order and predetermined pixels are turned on during each of the sub-frames.
When the n-number of sub-frame periods appear, one frame period terminates. At this time, the gray scale of a pixel is determined by cumulatively calculating the length of the display periods for which the pixel is turned on. For example, assuming that n =8 and the obtainable luminance in the case where the pixel emits light for all the display periods is 100%, a luminance of 75% can be represented if the pixel emits light during Ts1 and Ts2 and a luminance of 16% can be realized if Ts3, Ts5 and Ts8 are selected.
Incidentally, in the driving method of the time gray scale method which represents gray scales by inputting n-bit digital signals, the number of plural sub-frame periods into which one frame period is divided, and the lengths of the respective sub-frame periods and the like are not limited to the above-described examples.
With respect to the conventional OLED display device as described above, there are the following problems.
According to an OLED display device using an analog system, when an OLED is turned on, an analog signal voltage is inputted to a source signal line and applied to the gate of a driver TFT through a switch TFT so that a voltage between the gate and the source connected with a power supply line becomes Vgs. Thus, a drain current of the driver TFT is controlled to control a current flowing into the OLED.
Even if a voltage equal to Vgs is applied to the driver TFTs in a pixel portion, mobility, a threshold value, or the like of the TFTs are varied so that a drain currents are varied. As a result, there is a problem that display nonuniformity is caused in a display device, thereby reducing a display quality.
Also, according to an OLED display device using a time gradation system, a TFT is not operated in a saturation region but operated a linear region and Vds of the TFT is set to a small value, for example, 0.1 V so that a substantially entire voltage on a power supply line can be applied to an OLED. Therefore, the OLED is driven by a constant voltage without being influenced by a variation in TFTs so that a preferable image quality with high uniformity can be obtained in a display device. However, with respect to this system, there is the following problem. The characteristics of the OLED are being deteriorated while a current flows into it. With respect to the deterioration, there are two modes. A first mode corresponds to a reduction in light emission efficiency, that is, a phenomenon in which a brightness is reduced even if a constant current flows. A second mode corresponds to a problem in which a VF (forward voltage of the OLED) is risen. In the time gradation system, the second mode particularly becomes a problem. The OLED within a screen do not uniformly emit light at all times. Thus, a pixel with a high turning-on ratio and a pixel with a low turning-on ratio exist according to an image. In the case of the pixel with a high turning-on ratio, the deterioration rapidly progresses as a matter of course. Therefore, when a constant voltage is provided, the brightness of such a pixel is reduced. This is caused as a burn phenomenon so that the image quality of the display device is reduced.
As described above, even in either the analog system or the time gradation system, a problem is caused. In the case of the analog system, the improvement of characteristics of the OLED is required and it is important to conduct approaches with respect to materials. In addition, with respect to the improvement of the analog system, it is urgent to reduce a variation in TFT.
Also, even in a display device using an inorganic EL material, an FED display device, an electrophoretic display device, a liquid crystal display device, and the like, other than the display device using the OLED, a liquid crystal display device in which an analog buffer circuit and an DA converting circuit are incorporated is being developed as the display device is systemized. For the analog buffer circuit and the DA converting circuit, circuit precision is required. Thus, characteristics with small variations are required for the TFT.
When a display device using low temperature polysilicon is manufactured, it is general to crystallize an amorphous silicon film formed on a glass substrate by laser light. In the case of high temperature polysilicon using a quartz substrate, an amorphous silicon film is heated for thermal crystallization. However, the quartz substrate is expensive so that it is currently used for only a display device for projector having a small size. Thus, in order to provide a low cost display device, a glass substrate and laser crystallization are essential matters.
With respect to a laser oscillation device, there are a gas laser represented by an excimer laser, a solid laser represented by a YAG laser, and the like, which are used for crystallizing a surface layer of a semiconductor film in an extremely short time. The laser is broadly divided into two types, a pulse oscillation and a continuous oscillation according to an oscillation method. According to the pulse oscillation laser, its output energy is relatively high. Thus, when a size of a beam spot is set to several cm2, the productivity can be improved. In particular, when a shape of the beam spot is processed by using an optical system and thus made to a linear shape with a length of 10 cm or more, laser light irradiation to a substrate can be efficiently conducted to improve the productivity. Therefore, a pulse oscillation excimer laser is widely used for laser irradiation.
In recent years, it is found that a crystal grain size in a semiconductor film in the case where the continuous oscillation laser is used for laser crystallization becomes lager than that in the case where the pulse oscillation laser is used. For a semiconductor film having a large crystal grain size, the improvement of mobility and the reduction in variation resulting from the existence of a grain boundary are required.
However, with respect to a semiconductor film formed by laser crystallization using a laser, generally, a plurality of crystal grains are aggregated and formed, and these positions and sizes are random so that it is difficult to form a crystalline semiconductor film with designating a position and a size of a crystal grain. Thus, a grain boundary exists in a channel of a TFT. An infinite number of recombination centers and trapping centers resulting from the use of an amorphous structure, a crystal defect, and the like exist in the grain boundary as being different from the inner portion of the crystal grain. When a carrier is trapped in the trapping center, it is known that a potential of the grain boundary is increased and it becomes a barrier to the carrier, thereby reducing a current transport characteristic of the carrier. Therefore, when the grain boundary exists in the channel of the TFT, the mobility of the TFT is reduced, its threshold value is risen, or its off current is increased, that is, the characteristics of the TFT are greatly influenced. In addition, in each of a plurality of TFTs, the manner of existence of grain boundaries is completely independent. Thus, this causes a variation in characteristics of the TFTs. This variation causes a variation in on current in the above-mentioned driver TFT, thereby deteriorating the image quality of the display device.
The reason why positions of crystals obtained when laser light is irradiated to a semiconductor and sizes of grain boundaries become random is as follows. It takes some times to generate a solid phase nucleus in a liquid semiconductor film completely melted by laser light irradiation. Countless crystal nuclei are generated in a complete melting region with a lapse of time and then crystals grow from the respective crystal nuclei. Positions in which the crystal nuclei are generated are random so that they are nonuniformly distributed. Then, when crystal grains collide with each other, crystal growth is stopped. Thus, the positions and the sizes of crystals become random.
Accordingly, it is ideal that the influence of the grain boundaries is eliminated and a channel formation region by which the characteristics of a TFT are greatly influenced is composed of single crystals. However, it is difficult to crystallize a crystallized film in which no grain boundary exists by a conventional laser crystallization method.